"Shutting Down Your iMac
If you won’t be using your iMac for more than a few days, shut it down. Choose Apple () > Shut Down. To turn your iMac on again, press the power (®) button."

Therefore, for those who regularly use their iMac, leave it on all the time and put it to sleep overnight.

There is a practical reason for using the machine this way (or any item of electrical equipment). Continually turning it on and off can create undue strain on the components of the power supply caused by the initial power surge during switch-on.